By way of explanation, the electrical echo due to the two-wire/four-wire line transformers of the remote set, which is used to process the incident signals in order to make them compatible with the telephone line, perturbs the communications by sending over the line some of the signals received by the remote set. Likewise, in the case of a telephone set, an acoustic echo appears when the signal emitted by the loudspeaker, which corresponds to a signal spoken by the remote speaker, is picked up by the microphone.
For the user, these echoes then lead to the signal being reproduced in the earpiece with an offset relative to the transmission time. Such an offset becomes relatively problematic beyond the time when the delay in the system becomes large, i.e. for example more than 50 ms. The echo is generally reduced, or even cancelled, by fitting the sets with AEC (acoustic echo canceller) or EEC (electrical echo canceller) echo cancelling devices. Furthermore, access to a service or generally the establishment of communication between remote equipment may take place in various ways, i.e. by using the IP protocol, by using a switched telephone network, a GSM network, etc. These access modes may generate level variations in the signal delivered to the earpiece. This level variation may also be due to the speaker moving away from or closer to the microphone, as well as the natural variety of speech volume between individuals. However, the sound recording system has a finite dynamic range associated with the presence of an analogue-digital converter within the digital processing system fitted to the set. It is therefore necessary to keep the signals received from remote speakers within a certain range.
To this end, audio signal processing devices conventionally use an automatic gain control device AGC in order to keep the level of the output signal constant over a predetermined working range. The automatic gain control is based on using the energy of the signals and, by comparison with thresholds, employs either an amplification strategy or a strategy of attenuating the signal.
Such devices operate quite effectively. They nevertheless pose major problems when they are used in conjunction with an acoustic or electrical echo canceller. In fact, the automatic gain control means may be arranged either upstream or downstream of the echo cancelling means, considering the path of the received signal.
In general, when the automatic gain control means are arranged upstream of the echo cancelling means, it is very difficult to differentiate the echo reliably from the remote signal. In this case, operation may be perturbed depending on the orders of magnitude of the signals in the system.
In this case, concerning the electrical echo, although the automatic gain control means are liable to reduce the echo if it is large, especially in the case of an unmatched, “open” or “short-circuited” line, or if the remote signal is more than the maximum threshold of the automatic gain control means, they nevertheless have a certain number of drawbacks. This is because the automatic gain control means are driven both by the received signal coming from remote equipment and by the echo. They may therefore be perturbed by taking the echo signal into account. There is furthermore a risk of attenuating the signal from the remote equipment if the echo is large or, conversely, of amplifying the echo if this signal is weak. There is also a risk of amplifying the echo if it is weak. Lastly, the automatic gain control means which, by their nature, incorporate a nonlinear function, perturb the operation of the echo canceller which generally consists of an adaptive filter.
When the automatic gain control means are arranged downstream of the echo cancelling means, the echo signal is no longer capable of perturbing the automatic gain control, in so far as the signal coming from the remote equipment is essentially taken into account.
Furthermore, it does not perturb the echo canceller in so far as the nonlinearity lies downstream. Nevertheless, the automatic gain control means will then be perturbed by the processing operations carried out within the echo canceller, which also introduce a nonlinearity. Furthermore, the automatic gain control means are no longer capable of attenuating the echo.